Chap, iv.] THE ORBIT AND EYE. 51 



serrata. The optic nerve as it passes from the brain 

 receives its perineural sheath from the pia mater, and, 

 in addition, two other sheaths ; an outer from the 

 dura mater, and an inner from the arachnoid. These 

 sheaths remain distinct and separate, and the two 

 spaces enclosed may be injected, the outer from the sub- 

 dural, the inner from the subarachnoid space. Thus 

 inflammatory affections of the cerebral meninges can 

 readily extend along the optic nerve to the optic disc 

 through these spaces in the nerve sheath, while in 

 cases of intracranial disease other than meningeal. the 

 mischief may extend from the brain to the disc along 

 the interstitial connective tissue in the nerve. These 

 connections may serve in part to explain the frequent 

 association of optic neuritis with intracranial disease. 



The aqueous and vitreous humours. 

 The aqueous fills the space between the capsule and 

 suspensory ligament of the lens and the cornea. The 

 iris divides this space into two parts, the anterior 

 and posterior chambers. Since, however, the iris is 

 largely in actual contact with the lens, it happens 

 that the posterior chamber is represented by a little 

 angular interval between the iris, the ciliary processes, 

 and the zonula of the lens (Fig. 10). The cornea at 

 its circumference breaks up into bundles of fibres, 

 which are partly continued into the front of the iris, 

 and which constitute the ligamentum pectinatum 

 iridis. Between the processes of this ligament there 

 are intervals which lead into certain cavernous spaces 

 called the spaces of Fontana. These spaces in their 

 turn communicate with a large circular canal situate 

 in the sclerotic close to its junction with the cornea, 

 and known as the canal of Schlemm. This space is 

 in communication with the veins of the anterior part 

 of the sclerotic,* and thus through this somewhat 



* The precise manner in which these spaces communicate with 

 the neighbouring veins has not been demon stratedhistologically. The 

 statement is founded mainly upon the experiments of Schwalbe. 



